Crystal chemistry and thermoelectric properties of clathrates with rare-earth substitution

Based on an analysis of the geometric crystallographic relations a general classification 'scheme is presented for intermetallides with four-coordinated networks isomorphous with hydrate clathrates. We prepared novel europium substituted clathrates, Eu2-x(Sr,Ba)(6-x)MySi46-y (M = Al, Ga), consistent with the standardized clathrate I-Ba(8)AI(16)Ge(30) type structure (space group Pm3n). Europium. atoms in Ba compounds preferentially occupy the 2a position and thus form a new quaternary version of the Ba8Al16Ge30 structure type. All clathrates studied are metals with low electrical conductivity. The negative Seebeck coefficients indicate transport processes dominated by electrons as carriers. Eu2Ba6Al8Si36 and Eu2Ba6Ga8Si36 exhibit long-range magnetic order below 32 and 38 K of presumably ferromagnetic type. Magnetic susceptibilities indicate an EU2+ ground state, in fine agreement with L-III absorption edge spectra. (C) 2002 Elsevier Science B.V. All rights reserved.